yeah, you want the hole in really thin metal so that the light can enter and exit the hole at an extreme wide angle -- if it is there is practically no limit -- my ZeroImage 4 by 5 pinhole camera gets excellent coverage with a pinhole distance-to-film of 50 mm and even does ok at 25, although vignetting is rather extreme at that distance.

But I'd give it a shot in any event, the plastic in ur lens cap may be thin enough to give you good coverage with a .250 mm pinhole. 70mm is pretty far away.